Developments in or relating to drum rotors

ABSTRACT

A method of replacing a integrally bladed rotor section in a one-piece drum rotor, the method comprising removing the integrally bladed rotor section, replacing the integrally bladed rotor section with a replacement rotor section, and joining the replacement rotor section to the drum rotor by diffusion bonding under an axial contact pressure exerted between one or more respective pairs of bonding surfaces on the new rotor section and the drum rotor.

The present invention relates to a method of replacing a integrallybladed rotor section forming part of a one-piece drum rotor. Theinvention may be used to repair a damaged integrally bladed rotorsection, to remedy manufacturing defects in a particular integrallybladed rotor section, or to provide a “retro-fit” modification of aintegrally bladed rotor section in an existing rotor, for example tointroduce a design modification. An aspect of the invention relates tooriginal manufacture of a drum rotor by fabrication.

In the continuing drive to reduce weight in gas turbines, particularlyin aerospace applications, there has been a move towards usingintegrally bladed rotor technology to form integrally bladed disks. Aseries of integrally bladed rotors may be fusion welded to one anotherto form a one-piece drum rotor. Integrally bladed rotor manufacture isrelatively expensive, and therefore the resulting one-piece drum rotorstend to have a particularly high unit cost. Consequently, where a drumrotor suffers damage or other defect, such as manufacturing defects, itis often more cost-effective to modify or repair the drum rotor ratherthan scrap the drum rotor.

In practical terms, modification or repair of the drum rotor may requiresectioning and removal of one or more defective integrally bladed rotorsections, which are then replaced by new, replacement integrally bladedrotor sections. By “integrally bladed rotor section” is meant a bladedsection of the rotor i.e. an axial section of the rotor whichincorporates one or more rows of integral rotor blades. Although theterm “integrally bladed rotor” generally refers to a bladed disk, theterm is used in this specification to embrace similar components, suchas bladed rings or drums.

The new integrally bladed rotor sections are typically joined to theexisting drum rotor using electron beam welding, but the electron beamwelding process carries with it a number of disadvantages. The beamalters the cast microstructure of the material, and the materialproperties and stresses are also affected by an uneven thermal historyaround the circumference of the join and the variation in contractionagainst varying constaints. These and other factors make the rotorliable to strain-age cracking (particularly in the case of nickel-basedsuperalloys) and embrittlement which impact upon the life cycle of therotor. In addition, in order to limit the residual stress in the rotorcaused by electron beam welding, the rotor must typically be subjectedto a post-weld thermal treatment, which is costly. The weld profilecreated by electron beam welding may also be undesirable.

It is an object of the present invention to seek to provide an improvedmethod of replacing a integrally bladed rotor section in a one-piecedrum rotor.

According to the present invention, there is provided a method ofsecuring a rotor section in a one-piece drum rotor, the methodcomprising providing a rotor section, and joining the rotor section tothe drum rotor by diffusion bonding under an axial contact pressureexerted between one or more respective pairs of bonding surfaces on thenew rotor section and the drum rotor.

The rotor section may be a replacement rotor section and the methodfurther comprising the step of removing a prior rotor section from thedrum rotor and replacing the prior rotor section with the replacementrotor section.

The rotor section may be annular and may comprise a replacementintegrally bladed rotor. The rotor section may comprise a replacementintegrally bladed rotor and one or more spacer elements arranged axiallyin-between the replacement integrally bladed rotor and the drum rotor sothat each spacer element is thus adjacent to either the replacementintegrally bladed rotor, the drum rotor or another spacer element, eachspacer element further being joined to said adjacent spacer element,integrally bladed rotor or drum rotor by diffusion bonding under anaxial contact pressure exerted between a respective pair of bondingsurfaces on the spacer element and said adjacent integrally bladedrotor, drum rotor or spacer element. The rotor section may comprise aplurality of integrally bladed rotors so that each spacer element may beadjacent to any one of the replacement integrally bladed rotors.

The drum rotor and replacement rotor section may be sandwiched togetherthereby simultaneously to exert said axial contact pressure between eachpair of bonding surfaces.

The method may comprise application of a radial constraining pressurealong at least part of the rotor for controlling radial deformation ofthe drum rotor, for example buckling or compression-bulging of therotor.

In particular, the method may comprise:

-   i) interposing a powdered metal in-between one or more of the    respective pairs of bonding surfaces; and-   ii) locally heating the powdered metal under said axial contact    pressure to a sintering temperature, below the relevant melting    temperature of the drum rotor, thereby to sinter the powdered metal    and promote said diffusion bonding.

In one embodiment, the powdered metal is mixed with a binder and step i)comprises applying the resulting mixture directly to a bonding surface.In an alternative embodiment, the powdered metal is in the form of anannular pre-form insert and step i) comprises inserting the pre-forminsert between the bonding surfaces.

The powdered metal may be a powder or a partially consolidated powdermetal compact.

The pre-form insert may have an annular radius which is greater than theannular radius of the respective bonding surfaces in order to allow fora predetermined maximum radial shrinkage of the pre-form insert duringstep ii).

The respective bonding surfaces may be provided with one or moreco-operative locating features for locating the new section and drumrotor in a preferential, relative axial and/or circumferential alignmentprior to said diffusion bonding.

The local heating may be provided by one or more induction coils. Theremay be an in-process rotation or oscillation of the assembly of therotor section and the drum relative to the induction coils by rotatingeither the coils or the assembly so as to even out temperatures toaccommodate any variation in heating due to the location of the coilconnections (e.g. power and water cooling inputs/outputs).

The local heating may be performed in a vacuum less than or equal to1×10⁻⁵ Torr. The vacuum may be applied locally to the bonding surfaces.Where a local vacuum is applied, the vacuum sealing surfaces may becooled. Alternatively, the entire component may be subjected to thevacuum.

The powdered metal may be maintained at a temperature of 900-1000° C.for 1-2 hours during the sintering cycle. The bonding surfaces and theadjacent portions of the replacement rotor section and the drum rotormay be heated to reduce any quenching effect caused by the rotor sectionand the drum rotor, if cold, during the diffusion bonding process. Theheating could be supplied continuously or, alternatively, as a series ofheat pulses to allow heat dissipation. In one embodiment, a travellinghot spot could be provided. In particular, a travelling hot spot may beuseful where access to rotor section and the drum rotor is restricted.

In one exemplary embodiment, the axial contact pressure exerted betweenthe bonding surfaces is up to 10 MPa.

Though not essential, the drum rotor and new section may each beoriented such that the nominal centreline of the rotor is substantiallyvertical during diffusion bonding.

The drum rotor and the new section may each be formed from either atitanium-based or nickel-based super-alloy.

The method of replacing a integrally bladed rotor section according tothe invention may in particular be used in a method of repairing ormodifying a one-piece drum rotor in order to replace a damaged ordefective integrally bladed rotor section.

According to another aspect of the present invention, there is provideda method of fabricating a one-piece drum rotor, the method comprisingjoining two rotor sections of the drum rotor by diffusion bonding underan axial contact pressure exerted between respective bonding surfaces onthe rotor sections.

Embodiments of the invention will now be described by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a sectional view along the centreline of a one-piece drumrotor;

FIG. 2 is a sectional view corresponding to FIG. 1, but with aintegrally bladed rotor section removed;

FIG. 3 is an exploded sectional view of a one-piece drum rotor showingintroduction of a replacement integrally bladed rotor to be joined tothe drum rotor;

FIG. 4 is a sectional view illustrating diffusion bonding of thereplacement integrally bladed rotor and drum rotor shown in FIG. 3;

FIG. 5 is a sectional view corresponding to FIG. 4, showing the drumrotor following diffusion bonding of the replacement integrally bladedrotor;

FIG. 6 is a sectional view illustrating diffusion bonding of a separatespacer element in-between the replacement integrally bladed rotor andthe drum rotor; and

FIG. 1 shows part of a one-piece drum rotor having a nominal centreline(CL).

The drum rotor 1 comprises a plurality of integrally bladed rotors 3, 5,7, 9, 11 which are joined together by fusion welds 10 a, 10 b, 10 c, 10d (the reference numerals 10 a, 10 b, 10 c and 10 d are intended torepresent the heat affected zones associated with the respective fusionwelds; for clarity purposes, the scale of these heat affected zones isgreatly exaggerated in FIG. 1).

The drum rotor 1 may be formed from any suitable alloy, includingtitanium-based or nickel-based super alloys.

As previously mentioned, it may be desirable during the life cycle ofthe drum rotor 1 to replace one or more of the integrally bladed rotors3, 5, 7, 9, 11, for example because the integrally bladed rotor isdamaged or is otherwise defective.

FIGS. 2 to 5 illustrate replacement of the integrally bladed rotor 7 inaccordance with the present invention.

Firstly, a integrally bladed rotor section is removed from the drumrotor 1 using conventional techniques, as illustrated in FIG. 2. Theintegrally bladed rotor section incorporates the integrally bladed rotor7, as well as the fusion welds 10 b and 10 c either side of theintegrally bladed rotor 7. Removal of the integrally bladed rotorsection thus removes all of the original weld material and each heataffected zone 10 b and 10 c and creates two bonding surfaces 13 a, 13 bon the drum rotor 1; the bonding surfaces 13 a, 13 b are illustrated asbeing generally planar in FIG. 2, but this is not essential.

The bonding surfaces 13 a, 13 b should be as clean as possible, and ifrequired may be chemically cleaned using conventional techniques. Thebonding surfaces 13 a and 13 b may also be smoothed or polished toremove asperities, as required.

Following removal of the integrally bladed rotor section, a replacementrotor section, in this case a replacement integrally bladed rotor 15, isjoined to the drum rotor 1 by diffusion bonding, as illustrated in FIGS.3 and 4.

The replacement integrally bladed rotor 15 may correspond in compositionand/or configuration to the integrally bladed rotor 7. Alternatively,the replacement integrally bladed rotor 15 may have a differentconfiguration, or be formed from a different material to the removedintegrally bladed rotor 7, particularly in the case where replacement ofthe integrally bladed rotor is being carried out as part of a “retrofit” modification of the drum rotor 1.

The diffusion bonding process is illustrated in FIGS. 3 and 4. Powderedmetal in the form of a first preform powder ring 17 is interposedbetween the bonding surface 13 a on drum rotor 1 and a correspondingbonding surface 15 a on integrally bladed rotor 15. Similarly, powderedmetal in the form of a second preform powder ring 19 is interposedbetween the bonding surface 13 b on the drum rotor 1 and a correspondingbonding surface 15 b on the integrally bladed rotor 15.

The bonding surfaces 15 a and 15 b may be prepared in similar manner tobonding surfaces 13 a, 13 b using conventional techniques such aschemical cleaning and polishing to remove asperites etc.

The powdered metal used to form the preformed powder ring 17 and 19 maybe any suitable sinterable metal used in powder metallurgy such as iron,copper, nickel, beryllium, chromium, cobalt, molybdenum, stainlesssteel, tantalum, titanium, tungsten, and suitable alloys thereof.Preferably, the powdered metal has a composition which is the same as orsimilar to the composition of the replacement integrally bladed rotor 15and/or the drum rotor 1. The powdered metal may be formed usingconventional processes. A particularly suitable process is the PlasmaRotating Electrode Process (PREP). A conventional inert gas atomisationprocess may also be used; however, other processes may be used providedthey are suitable for producing clean, dry, unoxidised, spherical powderwith good packing characteristics and no, or minimal, internal porosity.

The size of powdered metal particles may vary, and blends of particlesizes may be used, but particle sizes less than 100 microns areconsidered to be particularly suitable.

The preform powder ring 17 and 19 may be partially consolidated, and maybe formed via conventional rolling or cold compression. Alternativelythe perform ring 17 and 19 may be fully consolidated.

Referring now to FIG. 4, each of the powder rings 17, 19 is locallyheated under an axial contact. pressure P between the respective bondingsurfaces 13 a, 15 a and 13 b, 15 b to sinter the powder preform ringsand promote diffusion bonding across the bonding surfaces 13 a, 15 a, 15b and 13 b.

The various bonding parameters may be controlled in accordance with thespecific composition of the preform rings 17, 19 and the integrallybladed rotors 5, 9 and 15. For integrally bladed rotors formed fromtitanium and nickel based super alloys, suitable joints may be obtainedusing bonding temperatures of 900 to 1000° C. for one to two hours,under an axial contact pressure up to 10 MPa. Temperatures outside thisrange may be suitable. For example, a temperature lower than 900° may beadequate if the axial contact pressure is high, or is maintained for alonger time.

In general, the sintering temperature required to sinter the powderedmetal is less than the melting temperature of the replacement integrallybladed rotor and drum rotor, so that the primary bonding mechanism isinter-atomic diffusion, either solid state diffusion or liquid phasediffusion, across the bonding surfaces. Thus, the finished joint willhave a high integrity and will not generally exhibit the heat affectedzones, and the grain coarsening within these zones, associated withfusion welding techniques such as electron beam welding (see FIG. 5).

During the sintering cycle, the preform rings 17, 19 may exhibitshrinkage, which will in general vary according to the residual porosityin the preform rings (the residual porosity will itself depend upon theprocess used to form the powdered metal, with gas atomisation processestending to result in higher residual porosities than the PREP process).In order to ensure full “wetting” of the bonding surfaces 13 a, 15 a, 15b and 13 b, the annular radii of the preform rings 17, 19 are thusdimensioned to exceed the annular radii of the bonding surfaces 13 a, 15a, 15 b and 13 b accordingly. Depending upon the actual shrinkage rateduring the sintering cycle, excess material 19 a may be left around thecircumference of the joint (see FIG. 5). The excess material 19 a mayact as a stress riser, in which case it may be desirable to remove theexcess material e.g. by subsequent machining.

It is envisaged that the preform rings 17, 19 may additionally act asspacer elements between the replacement integrally bladed rotor 15 andthe drum rotor, for example to off-set any difference in the relativeaxial lengths of the replacement integrally bladed rotor 15 and theremoved integrally bladed rotor section (the latter incorporating heataffected zones 10 b and 10 c).

One or more solid spacer elements may also be provided as part of thereplacement rotor section as required, inserted between the replacementintegrally bladed rotor 15 and the drum rotor 1. FIG. 6 shows a single,solid spacer element 21 inserted between the replacement integrallybladed rotor 15 and the drum rotor 1. The spacer element 21 is joined tothe replacement integrally bladed rotor 15 and the drum rotor 1 bydiffusion bonding in the manner described above, using an additionalpreform powder ring 25 in-between the replacement integrally bladedrotor 15 and the spacer element 21 (see FIG. 6).

The various bonding surfaces may be provided with one or more locatingfeatures for cooperative engagement to locate the replacement integrallybladed rotor 15 in preferential, relative axial and/or circumferentialalignment with the remainder of the drum rotor 1. Corresponding locatingfeatures may be provided on the preform powder inserts 17, 19 and/or 25as appropriate. Suitable locating features may include abutments,scallop features and chamfers; various other suitable locating featureswill be apparent to the skilled person.

The above-mentioned local heating may be provided by means of aninduction coil, preferably in the form of a graphite-lined inductionring, which may conveniently be configured to provide local heatingaround the full circumference of the rotor in the region of theinterface between bonding surfaces. Preferably, the induction coil iswater cooled. An induction coil should be provided in the region of eachpair of bonding surfaces, so that for example three separate inductioncoils would be required to provide suitable local heating in the case ofthe arrangement shown in FIG. 6. Although the use of an induction coilis considered to be particularly suitable, other local heating methodsmay however be used, for example resistance-heated thermal blankets,particularly where there is sufficient access to the interface betweenbonding surfaces.

The axial contact pressure between the various bonding surfaces canconveniently be applied and maintained simultaneously by clamping thedrum rotor 1 and replacement integrally bladed rotor 15 together in asuitable press. In this manner, the drum rotor and replacementintegrally bladed rotor are sandwiched together to generate the axialcontact pressure at the various bonding surfaces. Additional tooling maybe provided for exerting a radial isostatic constraining pressure on atleast part of the drum rotor 1, and possibly the entire length of thedrum rotor 1, in order to prevent or to control buckling, “compressionbulging” or other radial deformation of the drum rotor duringapplication of the axial contact pressure required for diffusionbonding. The constraining pressure may be applied on the inside and/oroutside of the drum rotor using suitable additional tooling. Suitablesensors may be used to provide feedback for corresponding closed-loopcontrol of the axial contact pressure and/or radial isostaticconstraining pressure as appropriate.

In one embodiment, the compression bulging can be controlled byregulating the bonding pressure. By appropriately controlling thebonding pressure, the radial thickness of the joint may be increased.Increased radial thickness of the joint can reduce joint stress and canallow for the joint to be machined or etched without reducing the radialthickness of the joint below the radial thickness prior to joining. Theshortening of the joined drum rotor 1, spacer element 21 and integrallybladed rotor 15 which occurs as a result of bulging may be accommodatedby adding length to the replacement integrally bladed rotor 15 and/orthe spacer element 21 prior to joining.

During diffusion bonding of the replacement rotor section to the drumrotor, the drum rotor is preferably orientated such that its nominalcentreline is substantially vertical, to limit the effect of gravity onaxial misalignment of the replacement integrally bladed rotor with thedrum rotor.

Alternative powder preform inserts other than a preform ring may beused. For example, the powdered metal may cold-rolled into a pre-formedfoil for insertion between bonding surfaces.

The use of a powder pre-form insert is not considered to be essentialand the powdered metal may be interposed between bonding surfaces usingany suitable method. For example, the powdered metal may be mixed with asuitable sacrificial binder and applied directly to one or both ofrespective bonding surfaces. Alternatively, the powdered metal may beapplied using a suitable cold metal spray process. A solid space elementmay be used to offset any loss in axial dimension not readilycompensated for by the axial thickness of the powdered metal applied tothe bonding surfaces.

Although in the embodiment described the one-piece drum rotor 1 isformed from a plurality of joined integrally bladed rotors, it isenvisaged that the invention might equally be used for removing aintegrally bladed rotor section from an integrally formed, one-piecedrum rotor i.e. a drum rotor which has initially been formed as a singlepiece, rather than by fusion welding together separate integrally bladedrotors.

It is envisaged that the replacement rotor section will comprise aintegrally bladed rotor, but this is not strictly essential. Forexample, the replacement rotor section may comprise one or more discs,rings or drums carrying separately-formed rotor blades (in which casethe resulting drum rotor will no longer be a one-piece drum rotor).

In a variant of the invention, the diffusion bonding process may beemployed in the original manufacture of a one-piece drum rotor to joinrotor sections of the drum rotor. In such applications, the method ofthe invention aids assembly of the component.

1. A method of securing a rotor section in a one-piece drum rotor, themethod comprising providing a rotor section, and joining the rotorsection to the drum rotor by diffusion bonding under an axial contactpressure exerted between one or more respective pairs of bondingsurfaces on the rotor section and the drum rotor.
 2. A method accordingto claim 1, wherein the rotor section is or comprises an integrallybladed rotor.
 3. A method according to claim 1, wherein the rotorsection is a replacement rotor section and the method further comprisingthe step of removing a prior rotor section from the drum rotor andreplacing the prior rotor section with the replacement rotor section. 4.A method according to claim 1, wherein the rotor section comprises areplacement integrally bladed rotor and one or more spacer elementsaxially in-between the replacement integrally bladed rotor and the drumrotor so that each spacer element is thus adjacent to either thereplacement integrally bladed rotor, the drum rotor or another spacerelement, each spacer element further being joined to said adjacentspacer element, integrally bladed rotor or drum rotor by diffusionbonding under an axial contact pressure exerted between a respectivepair of bonding surfaces on the spacer element and said adjacentintegrally bladed rotor, drum rotor or spacer element.
 5. A methodaccording to claim 4, wherein the rotor section comprises a plurality ofintegrally bladed rotors so that each spacer element may be adjacent toany one of the replacement integrally bladed rotors.
 6. A methodaccording to claim 1, wherein the drum rotor and rotor section aresandwiched together thereby simultaneously to exert said axial contactpressure between each pair of bonding surfaces.
 7. A method according toclaim 6, additionally comprising the application of a radialconstraining pressure along at least part of the rotor for controllingradial deformation of the drum rotor.
 8. A method according to claim 1,further comprising: i) interposing a powdered metal in-between one ormore respective pairs of bonding surfaces; and ii) locally heating thepowdered metal under said axial contact pressure to a sinteringtemperature, below the relevant melting temperature of the drum rotor,thereby to sinter the powdered metal and promote said diffusion bonding.9. A method according to claim 8, wherein the powdered metal is mixedwith a binder and step i) comprises applying the resulting mixturedirectly to a bonding surface.
 10. A method according to claim 8,wherein the powdered metal is in the form of an annular pre-form insertand step i) comprises inserting the pre-form insert between the bondingsurfaces.
 11. A method according to claim 10, wherein the pre-forminsert has an annular radius which is greater than the annular radius ofthe respective bonding surfaces in order to allow for a predeterminedmaximum radial shrinkage of the pre-form insert during step ii).
 12. Amethod according to claim 1, wherein respective bonding surfaces areprovided with one or more cooperative locating features for locating thenew section and drum rotor in a preferential, relative axial and/orcircumferential alignment prior to said diffusion bonding.
 13. A methodaccording to claim 8, wherein in step ii) the powdered metal ismaintained at a temperature of 900-1000° C. for 1-2 hours.
 14. A methodaccording to claim 1, wherein the axial contact pressure exerted betweenthe bonding surfaces is up to 10 MPa.
 15. A method according to claim 1,wherein, during said diffusion bonding, the drum rotor and new rotorsection are each oriented such that the nominal centreline of the drumrotor is substantially vertical.
 16. A method of replacing a blisksection in a one-piece drum rotor, the method comprising removing theblisk section, replacing the blisk section with a replacement rotorsection, and joining the replacement rotor section to the drum rotor bydiffusion bonding under an axial contact pressure exerted between one ormore respective pairs of bonding surfaces on the new rotor section andthe drum rotor.
 17. A method of fabricating a one-piece drum rotor, themethod comprising joining two rotor sections of the drum rotor bydiffusion bonding under an axial contact pressure exerted betweenrespective bonding surfaces on the rotor sections.